Porous ceramic layer system

ABSTRACT

A porous ceramic layer system is provided having two layers of porous layers with a tightly controlled and matched porosity. The layer system has a substrate, a metallic bonding layer on the substrate, an inner ceramic layer having a porosity, an outer ceramic layer on the inner ceramic layer, with a two-layered NiCoCrAlX layer as the metallic bonding layer, with a lower NiCoCrAlY layer, in which the content of chromium (Cr) in the lower NiCoCrAlY layer is less than the content of chromium (Cr) in the outer NiCoCrAlY layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to EuropeanApplication No. EP13183718 filed Sep. 10, 2013, incorporated byreference herein in its entirety.

FIELD OF INVENTION

The invention relates to a layer system having two different porousceramic layers.

BACKGROUND OF INVENTION

Ceramic protective layers are often used for components used at hightemperatures in order to protect the metallic substrate from relativelyhigh temperatures.

In this respect, the ceramic layers have a certain porosity in orderfirstly to reduce the thermal conductivity and in order to set a certainductility.

SUMMARY OF INVENTION

It is an object of the invention to optimize the thermal and themechanical properties.

The object is achieved by a layer system as claimed.

The dependent claims list further advantageous measures which can becombined with one another, as desired.

The ceramic and metallic layers ensure good oxidation protection andgood thermal insulation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 shows a layer system,

FIG. 2 shows a gas turbine,

FIG. 3 shows a turbine blade or vane,

FIG. 4 shows a combustion chamber,

FIG. 5 shows a list of superalloys.

DETAILED DESCRIPTION OF INVENTION

The description and the figures show merely exemplary embodiments of theinvention.

FIG. 1 schematically shows the layer system.

The layer system 1 is preferably a turbine blade or vane 120, 130 of aturbine, of a steam turbine, of a gas turbine 100 (FIG. 2), forstationary operation or for aircraft.

It is preferable for the substrate 4 to comprise a nickel-based orcobalt-based superalloy made of an alloy shown in FIG. 5. This ispreferably a nickel-based superalloy.

A metallic bonding layer 7, in particular of the MCrAl or MCrAlX type(M=Ni, Co, Fe, preferably Ni, Co), is preferably present on thesubstrate 4.

A ceramic layer 16 is applied to the metallic bonding layer 7, whereinan oxide layer (TGO) is either produced deliberately or applied betweenmetallic layers and ceramic layers or forms during the ceramic coatingor during operation of a layer system having the metallic layer 7.

The ceramic layer 16 has at least two, in particular only two, differentceramic layers 19, 22.

The lower ceramic layer 19 has a lower porosity than the outer ceramiclayer 22.

The porosity of the lower ceramic layer 19 is 8% to <15%, in particular11% to 13% (preferably % by volume).

It is preferable that the layer thickness of the inner ceramic layer 19is at least 10%, in particular 20%, very particularly 50% thinner thanthat of the outer ceramic layer 22.

The lower ceramic layer 19 has a thickness of 100±25 μm, whereas theouter ceramic layer 22 has a thickness of >100 μm.

The outer ceramic layer 22 has a porosity of >15% to 22%, in particularof 16% to 20%, and is preferably the outermost ceramic layer 22.

The material of the lower ceramic layer 19 is partially stabilized, inparticular by yttrium, zirconium oxide.

This material is preferably also used for the outer ceramic layer 22,although it is also possible to use a pyrochlore material, such asgadolinium zirconate.

The selection of the porosity of the ceramic layers surprisingly led toa longer service life compared to a layer of high porosity and equalthickness.

The fact that these elements of the metallic layer 7 also interact withthe base material of the substrate 4 owing to diffusion must also betaken into great consideration.

In general, it is assumed that, owing to the relatively greatinterdiffusion of chromium from the layer into the base material, whichgenerally has a lower chromium content than the layer, the differencebetween the chromium contents in the layer and the base material shouldnot be greater than approximately 5%. Otherwise, a more or less severeKirkendall porosity will arise, leading to premature failure of thelayer assembly with the base material. This has been confirmed by modelcalculations carried out appropriately. This behavior has been confirmedexperimentally, as proven by the comparison of layers having a lowchromium content and a high chromium content on IN 738 LC.

On the other hand, for the upper limit of the chromium content of thelayer, it should be taken into account that, given low chromium contentsof approximately 13% by weight chromium (Cr) in the layer, spinelformation with multiple cracking often arises at the surface, likewiseleading to a shortened service life of the protective layer system.Although a very balanced composition of the protective layer alreadyleads to good results, this does not yet constitute the optimum.

For the reasons mentioned above, a solution has been sought whichcombines all the advantages.

The solution proposed here presents a combination of layer compositionsas a duplex layer, which, compared to layer compositions to date, hasimprovements in terms of the aforementioned problems.

The assertions described are shown schematically and as metallographicimages in the enclosures.

What is proposed is a protective layer which, compared to the layersused to date, has better oxidation resistance and good thermomechanicalproperties and, on account of the substitution of rhenium, hasconsiderable cost benefits. In addition, the interdiffusion behavior issaid to be the same or better. In contrast to conventional layercompositions, the upper layer of the duplex layer has chromium contentsof >20% chromium, in particular >22% chromium (Cr). This avoids spinelformation and multiple cracking in the TGO. The higher chromium (Cr)content in the topmost layer has two reasons: on the one hand, despiteevaporation of chromium (Cr) during the solution annealing treatment,enough Cr remains present in the topmost layer in order to keep theactivity of aluminum high, and on the other hand the chromium serves asa nucleating agent for stable alpha-aluminum oxide.

The lower layer (boundary layer to the base material) of the duplexlayer has by contrast a considerably lower chromium content, preferablyof approximately 11% by weight-15% by weight chromium (Cr). Thisprevents a Kirkendall porosity which reduces service life at theinterface with the base material.

The other constituents of the layers are based on optimized proportionsof nickel (Ni), cobalt (Co), aluminum (Al), rare earth elements (Y, . .. ) and the like, but no rhenium (Re).

Example: Metallic duplex protective layer 7 comprises at least: a lowerNiCoCrAlY layer 10: an NiCoCrAlY protective layer having the composition(in % by weight) of Ni content: remainder cobalt (Co): 24%-26%, inparticular 25%, chromium (Cr): 11%-15%, in particular 13%, aluminum(Al): 10.0%-12.0%, in particular 11.0%, yttrium (Y): 0.2%-0.6%, inparticular 0.3% to 0.5%

Moderately high Co content: broadening of the beta/gamma field,avoidance of brittle phases

Moderate Cr content: low enough to avoid brittle phases (alpha-chromiumor sigma phase) and to avoid Kirkendall porosity and nevertheless topreserve the protective action over long periods of time

Moderately high Al content: sufficiently high to additionally deliver Alto preserve a stable TGO. Low enough to achieve good ductility and toavoid tendency toward embrittlement

Low Y content: sufficiently high to still form sufficient Y aluminatefor forming Y-containing pegs with low oxygen contamination low enoughto negatively accelerate the oxide layer growth of the Al₂O₃ layer, andalso an upper NiCoCrAlY layer (13): an NiCoCrAlY protective layer (13)having the composition (in % by weight) of Ni content: remainder cobalt(Co): 24%-26%, preferably 25%, chromium (Cr): 23%-25%, preferably 24%,aluminum (Al): 9%-12.0%, preferably 10.5%, yttrium (Y): 0.2%-0.6%,preferably 0.3%-0.5%

High Cr content: to avoid spinel and multiple cracking in the TGO andimprove the oxide layer formation of Al₂O₃ with low oxidation rates

Moderately high Al content: the Al content is lowered slightly comparedto the lower layer in order to minimize impairment of the ductility bythe high Cr content.

The NiCoCrAlY layers/alloys can also comprise further elements, other orfurther rare earth elements or Ta, Ti, Fe, . . . , but no rhenium (Re).

No chromizing of an individual layer is carried out for the upperNiCoCrAlY layer 13, and therefore there is also no chromium gradientpresent, because a uniform powder is used in order to apply the layer.

Thermodynamic phase calculations and also test results for therespective individual layer have shown that good results are present interms of oxidation, formation of the TGO and the mechanical properties.

The total layer thickness of the metallic layer 7 on the blade or vane120, 130 should preferably be 180 μm to 300 μm.

The lower metallic layer 10 is preferably sprayed with a fine powder andthe upper layer 13 consists of the powder having a high chromium contentwith a relatively coarse powder fraction, in order to provide not onlythe improved oxide layer formation but also the required high roughnessof R_(a)=9 μm to 14 μm for optimum bonding for a ceramic layer.

This procedure also has the advantage that no new cost-increasingprocess step is necessary.

FIG. 1 shows a layer system consisting of a substrate 4 and thetwo-layered NiCoCrAlY layer 7, which is composed of two different layercompositions 10, 13.

Nickel-based or cobalt-based superalloys, in particular alloys as shownin FIG. 3, can be used as the substrate 4.

FIG. 2 shows, by way of example, a partial longitudinal section througha gas turbine 100.

In the interior, the gas turbine 100 has a rotor 103 with a shaft whichis mounted such that it can rotate about an axis of rotation 102 and isalso referred to as the turbine rotor.

An intake housing 104, a compressor 105, a, for example, toroidalcombustion chamber 110, in particular an annular combustion chamber,with a plurality of coaxially arranged burners 107, a turbine 108 andthe exhaust-gas housing 109 follow one another along the rotor 103.

The annular combustion chamber 110 is in communication with a, forexample, annular hot-gas passage 111. There, four successive turbinestages 112 form the turbine 108, for example.

Each turbine stage 112 is formed, for example, from two blade or vanerings. As seen in the direction of flow of a working medium 113, in thehot-gas passage 111 a row of guide vanes 115 is followed by a row 125formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143,whereas the rotor blades 120 of a row 125 are fitted to the rotor 103for example by means of a turbine disk 133.

A generator (not shown) is coupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air135 through the intake housing 104 and compresses it. The compressed airprovided at the turbine-side end of the compressor 105 is passed to theburners 107, where it is mixed with a fuel. The mix is then burnt in thecombustion chamber 110, forming the working medium 113. From there, theworking medium 113 flows along the hot-gas passage 111 past the guidevanes 130 and the rotor blades 120. The working medium 113 is expandedat the rotor blades 120, transferring its momentum, so that the rotorblades 120 drive the rotor 103 and the latter in turn drives thegenerator coupled to it.

While the gas turbine 100 is operating, the components which are exposedto the hot working medium 113 are subject to thermal stresses. The guidevanes 130 and rotor blades 120 of the first turbine stage 112, as seenin the direction of flow of the working medium 113, together with theheat shield elements which line the annular combustion chamber 110, aresubject to the highest thermal stresses.

To be able to withstand the temperatures which prevail there, they maybe cooled by means of a coolant.

Substrates of the components may likewise have a directional structure,i.e. they are in single-crystal form (SX structure) or have onlylongitudinally oriented grains (DS structure).

By way of example, iron-based, nickel-based or cobalt-based superalloysare used as material for the components, in particular for the turbineblade or vane 120, 130 and components of the combustion chamber 110.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The guide vane 130 has a guide vane root (not shown here), which facesthe inner housing 138 of the turbine 108, and a guide vane head which isat the opposite end from the guide vane root. The guide vane head facesthe rotor 103 and is fixed to a securing ring 140 of the stator 143.

FIG. 3 shows a perspective view of a rotor blade 120 or guide vane 130of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plantfor generating electricity, a steam turbine or a compressor.

The blade or vane 120, 130 has, in succession along the longitudinalaxis 121, a securing region 400, an adjoining blade or vane platform 403and a main blade or vane part 406 and a blade or vane tip 415.

As a guide vane 130, the vane 130 may have a further platform (notshown) at its vane tip 415.

A blade or vane root 183, which is used to secure the rotor blades 120,130 to a shaft or a disk (not shown), is formed in the securing region400.

The blade or vane root 183 is designed, for example, in hammerhead form.Other configurations, such as a fir-tree or dovetail root, are possible.

The blade or vane 120, 130 has a leading edge 409 and a trailing edge412 for a medium which flows past the main blade or vane part 406.

In the case of conventional blades or vanes 120, 130, by way of examplesolid metallic materials, in particular superalloys, are used in allregions 400, 403, 406 of the blade or vane 120, 130.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blade or vane 120, 130 may in this case be produced by a castingprocess, by means of directional solidification, by a forging process,by a milling process or combinations thereof.

Workpieces with a single-crystal structure or structures are used ascomponents for machines which, in operation, are exposed to highmechanical, thermal and/or chemical stresses.

Single-crystal workpieces of this type are produced, for example, bydirectional solidification from the melt. This involves castingprocesses in which the liquid metallic alloy solidifies to form thesingle-crystal structure, i.e. the single-crystal workpiece, orsolidifies directionally.

In this case, dendritic crystals are oriented along the direction ofheat flow and form either a columnar crystalline grain structure (i.e.grains which run over the entire length of the workpiece and arereferred to here, in accordance with the language customarily used, asdirectionally solidified) or a single-crystal structure, i.e. the entireworkpiece consists of one single crystal. In these processes, atransition to globular (polycrystalline) solidification needs to beavoided, since non-directional growth inevitably forms transverse andlongitudinal grain boundaries, which negate the favorable properties ofthe directionally solidified or single-crystal component.

Where the text refers in general terms to directionally solidifiedmicrostructures, this is to be understood as meaning both singlecrystals, which do not have any grain boundaries or at most havesmall-angle grain boundaries, and columnar crystal structures, which dohave grain boundaries running in the longitudinal direction but do nothave any transverse grain boundaries. This second form of crystallinestructures is also described as directionally solidified microstructures(directionally solidified structures).

Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0892 090 A1.

The blades or vanes 120, 130 may likewise have coatings protectingagainst corrosion or oxidation e.g. (MCrAlX; M is at least one elementselected from the group consisting of iron (Fe), cobalt (Co), nickel(Ni), X is an active element and stands for yttrium (Y) and/or siliconand/or at least one rare earth element, or hafnium (Hf)). Alloys of thistype are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 orEP 1 306 454 A1.

The density is preferably 95% of the theoretical density.

A protective aluminum oxide layer (TGO=thermally grown oxide layer) isformed on the MCrAlX layer (as an intermediate layer or as the outermostlayer).

The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si orCo-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protectivecoatings, it is also preferable to use nickel-based protective layers,such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re orNi-25Co-17Cr-10Al-0.4Y-1.5Re.

It is also possible for a thermal barrier coating, which is preferablythe outermost layer and consists for example of ZrO₂, Y₂O₃-ZrO₂, i.e.unstabilized, partially stabilized or fully stabilized by yttrium oxideand/or calcium oxide and/or magnesium oxide, to be present on theMCrAlX.

The thermal barrier coating covers the entire MCrAlX layer.

Columnar grains are produced in the thermal barrier coating by suitablecoating processes, such as for example electron beam physical vapordeposition (EB-PVD).

Other coating processes are possible, e.g. atmospheric plasma spraying(APS), LPPS, VPS or CVD. The thermal barrier coating may include grainsthat are porous or have micro-cracks or macro-cracks, in order toimprove the resistance to thermal shocks. The thermal barrier coating istherefore preferably more porous than the MCrAlX layer.

The blade or vane 120, 130 may be hollow or solid in form.

If the blade or vane 120, 130 is to be cooled, it is hollow and may alsohave film-cooling holes 418 (indicated by dashed lines).

FIG. 4 shows a combustion chamber 110 of the gas turbine 100.

The combustion chamber 110 is configured, for example, as what is knownas an annular combustion chamber, in which a multiplicity of burners107, which generate flames 156, arranged circumferentially around anaxis of rotation 102 open out into a common combustion chamber space154. For this purpose, the combustion chamber 110 overall is of annularconfiguration positioned around the axis of rotation 102.

To achieve a relatively high efficiency, the combustion chamber 110 isdesigned for a relatively high temperature of the working medium M ofapproximately 1000° C. to 1600° C. To allow a relatively long servicelife even with these operating parameters, which are unfavorable for thematerials, the combustion chamber wall 153 is provided, on its sidewhich faces the working medium M, with an inner lining formed from heatshield elements 155.

Moreover, a cooling system may be provided for the heat shield elements155 and/or their holding elements, on account of the high temperaturesin the interior of the combustion chamber 110. The heat shield elements155 are then, for example, hollow and may also have cooling holes (notshown) opening out into the combustion chamber space 154.

On the working medium side, each heat shield element 155 made from analloy is equipped with a particularly heat-resistant protective layer(MCrAlX layer and/or ceramic coating) or is made from material that isable to withstand high temperatures (solid ceramic bricks).

These protective layers may be similar to the turbine blades or vanes,i.e. for example MCrAlX: M is at least one element selected from thegroup consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an activeelement and stands for yttrium (Y) and/or silicon and/or at least onerare earth element or hafnium (Hf). Alloys of this type are known fromEP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

It is also possible for a for example ceramic thermal barrier coating,which consists for example of ZrO₂, Y₂O₃-ZrO₂, i.e. unstabilized,partially stabilized or fully stabilized by yttrium oxide and/or calciumoxide and/or magnesium oxide, to be present on the MCrAlX.

Columnar grains are produced in the thermal barrier coating by suitablecoating processes, such as for example electron beam physical vapordeposition (EB-PVD).

Other coating processes are possible, e.g. atmospheric plasma spraying(APS), LPPS, VPS or CVD. The thermal barrier coating may include grainsthat are porous or have micro-cracks or macro-cracks, in order toimprove the resistance to thermal shocks.

Refurbishment means that after they have been used, protective layersmay have to be removed from turbine blades or vanes 120, 130 or heatshield elements 155 (e.g. by sand-blasting). Then, the corrosion and/oroxidation layers and products are removed. If appropriate, cracks in theturbine blade or vane 120, 130 or the heat shield element 155 are alsorepaired. This is followed by recoating of the turbine blades or vanes120, 130 or heat shield elements 155, after which the turbine blades orvanes 120, 130 or the heat shield elements 155 can be reused.

1-18. (canceled)
 19. A layer system at least comprising: a substrate, ametallic bonding layer on the substrate, an inner ceramic layer having aporosity, in % by volume, of 8% to <15% on the bonding layer, and anouter ceramic layer, having a porosity of >15% to 22%, on the innerceramic layer, a two-layered NiCoCrAlX layer as the metallic bondinglayer, with a lower NiCoCrAlY layer, in which the content of chromium(Cr) in the lower NiCoCrAlY layer is at least 3% by weight less than thecontent of chromium (Cr) in an outer NiCoCrAlY layer.
 20. The layersystem as claimed in claim 19, wherein the material of the lower ceramiclayer comprises zirconium oxide.
 21. The layer system as claimed inclaim 19, wherein the outer ceramic layer comprises zirconium oxide. 22.The layer system as claimed in claim 19, wherein the substrate comprisesa nickel-based or cobalt-based superalloy.
 23. The layer system asclaimed in claim 19, wherein the material of the ceramic layers isdifferent.
 24. The layer system as claimed in claim 19, wherein theinner ceramic layer has a form which is at least 10% thinner than theouter ceramic layer.
 25. The layer system as claimed in claim 19,wherein the layer system consists of: the substrate, the two-layeredmetallic bonding layer, the inner ceramic layer, and the outer ceramiclayer.
 26. The layer system as claimed in claim 19, wherein the contentof cobalt (Co) in the lower NiCoCrAlY layer is the same as or comparablewith the content of cobalt (Co) in the outer NiCoCrAlY layer.
 27. Thelayer system as claimed in claim 19, wherein the difference in thecontent of chromium (Cr) in the metallic layers is 3% by weight to 13%by weight.
 28. The layer system as claimed in claim 19, wherein thecontent of aluminum (Al) in the lower NiCoCrAlY layer is the same as orcomparable with the content of aluminum (Al) in the outer NiCoCrAlYlayer.
 29. The layer system as claimed in claim 19, wherein the contentof yttrium (Y) in the lower NiCoCrAlY layer is the same as or comparablewith the content of yttrium (Y) in the outer NiCoCrAlY layer.
 30. Thelayer system as claimed in claim 19, wherein the lower NiCoCrAlY layerhas the following composition in % by weight: cobalt (Co): 24%-26%,chromium (Cr): 11%-15%, aluminum (Al): 10.0%-12.0%, yttrium (Y):0.2%-0.6%, nickel.
 31. The layer system as claimed in claim 19, whereinthe upper NiCoCrAlY layer has the following composition in % by weight:cobalt (Co): 24%-26%, chromium (Cr): 23%-25%, aluminum (Al): 9%-12.0%,yttrium (Y): 0.2%-0.6%, nickel.
 32. The layer system as claimed in claim19, which has no gradients in the content of chromium (Cr) in the layer.33. The layer system as claimed in claim 19, wherein a thermally grownoxide layer is formed or is present on the outer NiCoCrAlY layer. 34.The layer system as claimed in claim 19, wherein the metallic bondinglayer has a total thickness of 180 μm to 300 μm.
 35. The layer system asclaimed in claim 19, wherein the powder used for the upper NiCoCrAlYlayer is coarser than the grain size of the powder for the lowerNiCoCrAlY layer, such that the upper layer comprises larger grains thanthe lower layer.
 36. The layer system as claimed in claim 19, whichcomprises no rhenium (Re) in the metallic layers.